366 HOW CEOPS GKOW. 



those of the storiiata excepted. Sachs found that solution 

 of indigo- quickly entered the roots of a seedling bean, 

 but required a considerable time to penetrate the stem, (p. 

 239.) Hallier, in his experiments on the absorption of 

 colored liquids by plants, noticed in all cases, when leaves 

 or green stems were immersed in solution of indigo, or 

 black-cherry juice, that these dyes readily passed into and 

 colored the epidermis, the vascular and cambial tissue, 

 and the parenchyma of the leaf-veins, keeping strictly to 

 the cell-walls, but in no instance communicated any color 

 to the cells containing chlorophyll. {Phytopathologie, 

 Leipzig, 1868, p. 67.) We must infer that the coloring 

 matters either cannot penetrate the cells that are occupied 

 with chlorophyll, or else are chemically transformed into 

 colorless substances on entering them. 



Sachs has shown in numerous instances that the juices 

 of the sieve-cells and cambial tissue are alkaline, while 

 those of the adjoining cell-tissue are acid when examined 

 by test-paper. [Exp. Phys. der Pflamen, p. 394.) 



When young and active cells are moistened with solu- 

 tion of iodine, this substance penetrates the cellulose 

 without producing visible change, but when it acts upon 

 the protoplasm, the latter separates from the outer cell- 

 wall and collapses towards the center of the cavity, as if 

 its contents passed out, without a corresponding endos- 

 mose being possible, (p. 224.) 



We may conclude from these facts that the membranes 

 of the cells are capable of effecting and maintaining the 

 separation of substances which have considerable attrac- 

 tions for each other, and obviously accomplish this result 

 by exerting themselves superior attractive or repulsive 

 force. 



The influence- of the membrane must vary in character 

 with those alterations in its chemical and structural consti- 

 tution which result from growth or any other cause. It is 

 thus, in part, that the assimilation of external food by the 



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